What happens to clay in a kiln?
Firing clay is where the magic happens — and the science.
As heat builds, your humble lump of earth transforms into something permanent, functional, and often beautiful. But what’s actually going on inside that kiln?
From drying to vitrification, each stage plays a critical role. In this guide — informed by trusted sources like Ceramic Arts Network and The Pottery Wheel — we break down what happens to your clay as it travels through fire.
Here’s a step-by-step look at how clay transforms as the temperature rises.
As temperature rises, clay loses water, burns off organics, and undergoes chemical and structural changes that harden it into ceramic.
Stage 1: Drying (Below 100°C)
Even if your clay feels dry, it still contains physical water — water that must evaporate slowly.
🔥 Why it matters:
If the kiln heats too fast at this stage, the water turns to steam too quickly, and your pot can crack or explode.
Stage 2: Burn-off (Around 350–600°C)
Now the clay releases chemical water and begins burning off organic materials like paper, grog additives, or leftover plant matter.
🔥 Why it matters:
This is the smelly part of a bisque firing. Proper venting is essential!
Stage 3: Quartz Inversion (573°C)
Quartz particles inside the clay undergo a structural change at 573 °C — known as quartz inversion. This sudden shift can cause cracks if the kiln heats too quickly. [Ceramic Arts Network]
🔥 Why it matters:
Heat too fast through this stage, and you'll risk cracks, warping, or breakage.
Want to see this in action?
When you join a class at Mayfield Studios, we don’t just teach you how to throw — we show you what happens inside the kiln. From dry clay to vitrified stoneware, you’ll witness the transformation for yourself.
👉 Book a class today
Stage 4: Sintering Begins (700–900°C)
The clay particles start bonding slightly, forming a solid matrix.
It’s not yet waterproof — but it holds together.
🔥 Why it matters:
Your pot is now no longer just dried mud — it’s officially ceramic.
Stage 5: Vitrification (1000–1300°C, depending on clay)
As temperatures rise above 1000 °C, the clay begins to vitrify — that is, to fuse into a glass-like solid. This process seals the body, making it water-tight and durable. [The Pottery Wheel]
🔥 Why it matters:
Each clay has its vitrification point. Firing too low = weak and absorbent.
Too high = warped, bloated, or melted.
So... What Are You Left With?
A porous but strong bisque if stopped early (Cone 06–04)
A vitrified, water-tight piece if glaze-fired at Cone 6 or 10
A completely changed material: from mud to stoneware, forever
Understanding the stages of firing doesn’t just help you avoid cracks — it deepens your relationship with clay. Whether you're a beginner or building a body of work, firing is where your intentions become durable.
Want to learn how to fire your own work or see a kiln in action? Explore our pottery classes in Melbourne and discover the full journey — from wheel to warmth, fire to form.
References:
Hamer, F. & Hamer, J. (2004). The Potter’s Dictionary of Materials and Techniques. A&C Black.
Digitalfire. “Firing Schedule.” digitalfire.com
Ceramic Arts Network. “What Happens During Firing.” ceramicartsnetwork.org
